Electrochemical reduction of CO2 to HCOO- and CO on cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane) based molecular catalysts has been studied using the computational method within framework of density functional theory. The stable conformations of cyclam (trans I and III) with Cr, Mn, Fe, Co, Ni, Cu and Zn as centers were examined using water as an implicit solvent. The reaction can happen through two cycles; LM2+ +e- → LM+ (+ H+) → LMH2+ (+e- ) → LM-H+ (+CO2 or H+) → LM2+ + HCOO- or H2, (L = cyclam, M = metal center) (hydride donor cycle) and; LM2+ +e- → LM+ (+ CO2) → LMCO2+ (+ e- ) → LMCO20 (+ H+) → LMCO2H+ (+ H+) → LM2+ +CO+H2O (CO cycle). The metal center in the complex must be reduced first. Protonation of reduced metal center (LM+) results in metal-hydride (LM-H+) which will drive the reaction to form either HCOO- or H2. Alternatively, the reduced metal center reacts with CO2 to form a CO2 adduct (LMCO2+) which can be further reduced to CO. Based on the calculated limiting potential and pH to generate LM-H+ and pH to suppress hydrogen evolution (H2) we determined that trans I conformation with Ni center is selective to HCOO-; trans I conformation with Cu center is at borderline of HCOO- and H2 selectivities; trans III conformations with Ni and Cu centers and both conformations of cyclam with Co and Zn centers are selective to hydrogen gas. Both conformation of cyclam with Cr, Mn and Fe centers cannot form stable metal-hydride and will not have HCOO- or H2 selectivity, instead, they are CO selective. The limiting potential, pKa of LMCO2H+, binding energy of CO2, as well as desorption energy of CO can be used to predict the efficiency of these catalysts for CO formation.

Electrochemical reduction of carbon dioxide to CO and formate with [(cyclam)M]2+ catalysts (M= Cr, Mn, Fe, Co, Ni, Cu and Zn) in water: DFT study

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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